1. Field of the Invention
The present invention relates to an optical system having an image rotating function, and more particularly to an optical system having an image rotating function mounted in an optical apparatus, such as a film camera, a television camera, or a video camera on an image side with respect to an image pick up lens thereof.
2. Related Background Art
As a means for rotating an image in an image pick up optical system or an observation optical system, there has been known a method of disposing an image rotator in an optical path of an optical system. The image rotator includes odd-numbered reflection surfaces and has an incident optical axis and an emission optical axis made coaxial to each other. When the image rotator is rotated with an optical axis of the image pick up optical system as a rotation axis by a predetermined angle, the image is rotated by twice the predetermined angle.
FIG. 20 is a conceptual diagram of such an image rotator. As a related art example where the image rotator is disposed in an observation optical system, Japanese Patent Application Laid-open No. 2002-014283 discloses an observation optical system in which an image rotator prism is disposed, and the image rotator prism is used to correct an image rotation accompanying a rotational movement of a rotating mirror.
Further, Japanese Patent Application Laid-open No. S50-034552 discloses an image pick up optical system provided with a relay optical system having an image rotator disposed on an image side with respect to an image pick up lens, and rotating a first image of the image pick up lens to effect reimaging.
When the above-mentioned image rotators are used, there is a demand to rotate an image in real time for achieving a special effect in photography. However, an image pick up lens to be used varies depending on photograph application, and may be a wide angle lens, a telescopic lens, a zoom lens, or the like. Therefore, to cope with the above demand, mounting an optical system having an image rotating function to an existing lens as an adapter results in high versatility and is more advantageous than the case where a specific lens is prepared for inserting an image rotator in an observation optical system as described in Japanese Patent Application Laid-open No. 2002-014283.
In addition, regarding an arrangement position of an optical system for image rotation in an image pick up optical system, to avoid an influence from change in viewing angle of an image pick up lens or change in viewing angle due to zooming, an image rotator is preferably disposed on an image side with respect to an image pick up lens as disclosed in Japanese Patent Application Laid-open No. 50-034552. At this time, it is preferable in practical use that degradation in optical performance of an image be small, a field angle and an F number intended by a photographer be obtained, quantity of light in periphery be secured, and the like. Furthermore, it is important for an entire apparatus to be compact from a practical standpoint.
Incidentally, in Japanese Patent Application No. S50-034552, a field lens is disposed on a first imaging plane of an image pick up lens, its optical power is appropriately controlled, and a pupil of a relay lens is set to be disposed in a vicinity of a center of an image rotator prism, thereby realizing the compact image rotator prism. However, according to such a structure, when an on-axis or off-axis marginal ray having high divergence characteristic enters the image rotator prism, there is a problem in that eclipse occurs in mid-course of an optical path within the image rotator. Therefore, the prism needs to be made large in order to avoid the eclipse, which is disadvantageous to produce a compact optical system.
Also, in Japanese Patent Application Laid-open No. S50-034552, the image rotator is constituted by a prism, and therefore there is a problem in that various kinds of aberration are generated in the image rotator, degrading its optical performance. When an attempt is made to correct the aberrations, a large number of lenses for correcting those aberrations are required, which is disadvantageous to produce the compact optical system.